Electric field Physics 122 11/18/2018 Lecture II

Workshops We are adding new workshop times as we speak. I’ll send email today to everyone once they are open. 11/18/2018 Lecture II

Website Both workshop modules and homework problems are from Giancoli Different, but deal with the same concepts Workshop problems are a bit easier, helpers for your homework problems Questions and problems are assigned http://www.pas.rochester.edu/~regina/PHY122/SCHF05.html 11/18/2018 Lecture II

Lab sign up Labs start this week! Remember to do a little homework from lab manual before the first class Sign up at http://www.pas.rochester.edu/~physlabs/ In case of problems send email to physlabs@pas.rochester.edu 11/18/2018 Lecture II

Electric field 11/18/2018 Lecture II

Concepts Primary concepts: Secondary concepts: Electric field Field lines Shields 11/18/2018 Lecture II

Laws Superposition principle 11/18/2018 Lecture II

Skills Calculate electric field of a system of charges 11/18/2018 Lecture II

Coulomb’s law + 1 2 1 2 - + 11/18/2018 Lecture II

System of charges Calculate the net electrostatic force on particle 3. Input: Q1= - 86mC= - 86.10-6 C Q2= + 50mC= + 50.10-6 C Q3= + 65mC= + 65.10-6 C r13=60cm=0.60m r23=30cm=0.30m What if the value of Q3 changes? 11/18/2018 Lecture II

Electric field + + F – force between two charges(N) Q – electric charge (C= Coulomb) E – electric field created at point 1 by charge 2 Charge 2 has changed the property of space at point 1 Charge 1 is experiencing this change + + 1 2 11/18/2018 Lecture II

Electric field E – electric field is the force F exerted on a small positive test charge, divided by the magnitude of this charge q. Electric field is a vector. Electric field is measured in N/C. 11/18/2018 Lecture II

Electric field Electric field is like the ski slope Charge – a skier The slope is there whether you ski down or not 11/18/2018 Lecture II

Field lines Field lines at each point are parallel to the electric field at this point. Let’s take a sample (+) charge and examine electric field pattern around a point positive charge. Like signs repel. Field lines are going radially away from the positive charge + + + + + + + + + 11/18/2018 Lecture II

Field lines Field lines at each point are parallel to the electric field at this point. Let’s take a sample (+) charge and examine electric field pattern around a point negative charge. Unlike signs attract. Field lines are going radially toward the negative charge + + + - + + + + + 11/18/2018 Lecture II

Superposition of fields Principle of superposition: Net field created by a system of charges is a vector sum of fields created by individual charges: Positive test charge + + - 1 2 11/18/2018 Lecture II

Rules of field lines Field lines Are parallel to the electric field Point away from positive charges Point toward negative charges Never cross Can be open ( go to infinity) Their density represents the intensity of the electric field. 11/18/2018 Lecture II

Constant field -Q +Q Electric field is constant between two very large parallel plates, one with a positive charge +Q, the other one with a negative charge - Q 11/18/2018 Lecture II

Charges in electric fields Positive charges experience force along the direction of the field. Negative charges – against the direction of the field. + - 11/18/2018 Lecture II

Electric field in conductors Conductor – a material with abundant free (to move) charge E=0 in good conductors in the static situation. E is perpendicular to the surface of conductor. Metal hollow boxes are used to shield electric fields. + + + + + - - - - - 11/18/2018 Lecture II

Test problem #4 Points A, B and C are at various distances from a given point charge.Which statement is most accurate? The electric field strength is A Greatest at point A. B Greatest at point B. C Greatest at point C. D The same at all three points. 11/18/2018 Lecture II

Test problem #5 Which diagram best represents the electric field lines around two oppositely charged particles? A - C B - D 11/18/2018 Lecture II

Test problem #1 Two point charges are separated by 10meters. If the distance between them is reduced to 5.0meters, the magnitude of the electrostatic force exerted on each charge A decreases to half its original value. B increases to twice its original value. C decreases to one quarter of its original value. D increases to four times its original value. 11/18/2018 Lecture II

Test problem #2 If the magnitude of the charge on two identical charged bodies is doubled, the electrostatic force between the bodies will be A doubled. B halved. C quadrupled. D unchanged. 11/18/2018 Lecture II

Test problem #3 If the charge of one point charge is doubled and that of the other charge stays the same, the force will be A doubled. B halved. C quadrupled. D unchanged. 11/18/2018 Lecture II

Motion in electric field Remember F=ma ? This tells about the effect the force will have on an object with mass m In electric field F=qE This tells you about the amount of force electric field exerts on the object Answer the question: what is the origin of forces? E v d E=2.0x104N/C e=1.6x10-19C m=9.1x10-31kg d=1.5cm=0.015m v=? 11/18/2018 Lecture II

Motion in electric fields a is always || to net F Remember v is not always || to a (projectile, circular motion) - - - - vx vy + + + + vx=const vy=at=tF/m=tEq/m 11/18/2018 Lecture II